Well, to be fair, the very last part of this page discusses the 'why not open source' (in bold print):
It is important to note that we do release the scientific modifications back to the open source community, but do not release information which would enable donors to cheat on points, which some donors have done ruining the experience for many others.
So, basically, they are worried about cheating. It's more or less the same issue MegaGlest would need to deal with when it comes to competitive player rankings.
Sadly, their latest release seems to date back to 2013, which means it is probably full of (now) known bugs with security impact. And the fact that the software is closed souirce means only they could provide fixed builds (which they have not, but the software is still being distributed). If someone identifies a network exploitable security hole in this software any client running it could be added to a newly formed botnet.
So please don't get me wrong: I'm totally in favor of science, and think we should all contribute to it where we can. But since science is (or should be) publicly funded (i.e. by the people) already, I personally think that anything they produce should also be returned to the general public (including full source material). Doing this would be a relevant contribution to a more balanced world (where education is accessible to more people, and actual competition is revived - within limits).
This said, I do not and did not mean to discourage anyone using this or similar software (despite for the security aspect maybe, but it's obvious and everyone can just make up their minds on it).